First NAS was a Buffalo TeraStation, with a standard firmware just hacked (to get ssh access through Dropbear and to thus be able install Firefly). Tended to crash quite often though, and imho the Uboot loader wasn’t available back then and the box was needed for something else (may try Debian on the same machine at some later point).
Then I got myself a Buffalo Kurobox just to be a music NAS. Thanks to Debian, UTF-8 isn’t a problem (just had to set the locale). Speed is just fine (a bit slow for Slimserver, but very fast for Firefly).